Permanent childhood hearing loss, which occurs in 1-2/1000 live-births, has negative impacts on children's lives, at high societal cost. Universal newborn hearing screening (UNHS) offers hope for early treatment. However, UNHS is costly, and evidence of its effectiveness in improving long-term outcomes, at a population level, was lacking. By capitalizing on a unique service and research environment in Australia - where children access uniform hearing services from a single provider (Australian Hearing, AH) but had differential access to UNHS during a narrow time window - we addressed the evidence gap by conducting the Longitudinal Outcomes of Children with Hearing Impairment (LOCHI) study. The funding from NIH enabled us to assemble a population-based cohort of 450 children with significant bilateral hearing loss (53% fitted with hearing aids before 6 months of age). We measured their outcomes from diagnosis to 5 years old. We found that early implantation was positively linked to better outcomes for profound loss, but early hearing aid fitting had limited gains for children with less severe loss. As the cohort ages, the effectiveness of UNHS may alter as benefits in additional domains may be measured. In showing the limited gains made by UNHS and early fitting alone, our findings galvanize new research into how to effectively intervene to capitalize on early treatment for improving outcomes. This application is for Phase 2 of the LOCHI study. The objectives are to investigate the efficacy of early intervention for improving long-term population outcomes, across the entire spectrum of hearing loss: children with mild or unilateral hearing loss (MUHL), which occurs in 1-2/1000 live-births, are also identified via UNHS but for whom audiological treatment is controversial. We propose to measure outcomes of the LOCHI cohort at age 9 years in an expanded range of domains; and to include a randomized controlled trial of intervention for a new cohort of children with MUHL. Our specific aims are: 1) describe the speech, language, literacy, mental health and quality of life of children who received early or later intervention; 2) identify modifiable factors that influence performance outcomes and standard score trajectories; 3) determine predictability of early outcomes for later development to identify the critical age at which children with poor outcomes at age 9 years can be reliably detected; and 4) investigate cognitive abilities and learning strategies of children. All outcomes data and a wide range of predictors will be evaluated in multiple regression analyses. The data and results will be incorporated in a software tool that we will develop to facilitate clinical use. As the research arm of AH, we will minimize loss to follow up by maintaining contact via the service provision network. Achieving the aims will provide the evidence base to 1) guide best-practice intervention following diagnosis across the spectrum of hearing loss; and 2) demonstrate outcomes affected by access to early intervention.